The extent to which early experience with flavors, especially taste and odors perceived retronasally, influence later flavor preferences and food choice in humans remains a mystery. The investigators propose a series of studies to investigate this important issue by using as a model system a class of infant formulas that are hydrolyzed protein based, and thus have very pronounced and distinctive flavors which are unpalatable to older infants, children and adults. Recent studies in the investigators' laboratory provided the first experimental demonstration that infants younger than 4 months of age willingly accept substantial amounts of, and satiate while feeding, a novel, protein hydrolysate formula. Moreover, it is well-known, although there are no direct experiments demonstrating it, that if these formulas are introduced to infants within the first few months of life and are continued to be fed, they remain highly acceptable at least until one year of age or older. This implicates a sensitive period during which exposure to the formulas, which are unpalatable to adults and infants older than 4 months of age without exposure, renders them acceptable and presumably palatable. It is the goal of this project to investigate the amount and timing of exposure to a protein hydrolysate formula necessary to render it acceptable at a later age (Aim 1); to determine whether experience with hydrolysate formulas impacts upon flavor and food preferences during early and late childhood (Aim 2); to investigate whether experience with a particular flavor in a milk-based formula would heighten the acceptance of a similarly flavored protein hydrolysate formula in infants who have not been previously exposed to hydrolysates (Aim 3); and to investigate the chemosensory attributes responsible for the age-related difference in the acceptance of protein hydrolysate- containing formulas (Aim 4). These experiments will provide new information on this dramatic developmental change in the sensory-guided responses of human infants, and more generally, will provide the first sustained investigation on the impact of early flavor experience on later acceptance and food preferences in human infants and children and will thus provide fundamental insight into this important, yet under- investigated, research area.